IS HYDRAULIC OIL CAN BE USE AS REPLACEMENT FOR GEAR OIL?

Answer 1

No not unless your trying to ruin the gearbox its not advisable you do that 

Answer 2

No. Gear oil has completely different anti-wear additives for the ring and pinion and or standard transmission.

Answer 3

No.  Gear oil is needed to reduce friction.  Hyd oil won't do that.

Answer 4

Only in older motorcycles can you use transmission fluid in suspension forks. 

Answer 5

No. Unless you wish to quickly destroy the transmission.

Answer 6

Hydraulic oil would likely be far too thin (low weight oil) and wouldn't stick to the gears well enough to lubricate them.  If you're in a pinch and need to fill something with gear oil, you can use chainsaw bar oil which is quite thick and tacky similar to 80 or 90 weight gear oil, or Lucas oil additive which is very thick and tacky, or even 40 weight motor oil.  But you'll definitely want to replace it with the proper gear oil once you get a supply of the stuff.  Hydraulic oil is thin like power steering fluid or transmission fluid.

Answer 7

No